Driveability Problems, Hard Start, and DTC P1406 is Set
File In Section: 6E - Engine Fuel & EmissionBulletin No.: 66-65-07A
Date: July, 1998
Subject:
Engine Surge, Knock, Rough Idle, Stall, Hard Start, DTC P1406 (Flash Calibration)
Models:
1995 Chevrolet and GMC S/T Models
with VCMX-A Controlled 4.3L Engine (VIN Code W - RPO L35)
This bulletin is being revised to update the initial service calibration information that may set a false DTC P0401 and to add new CD ROM disk information. Please discard Corporate Bulletin Number 66-65-07 (Section 6E - Engine Fuel & Emission).
Condition
Some owners with a VCMX-A Equipped 4.3L Engine may experience Linear EGR valve sticking. This may cause DTC 1406 to be set which may be accompanied by poor driveability condition(s) such as engine surge, knock on moderate acceleration, engine stall, rough idle and/or hard starting. Vehicles that have been previously updated per the original bulletin may set a false DTC P0401. The new engine calibrations listed should eliminate this false code. The VCMX-A controller is located under the hood on top of the coolant bottle and can be identified by its cooling fins.
Cause
The Linear EGR valve may be sticking because carbon deposits on the valve prevent the valve from responding to VCM system commands.
Correction
Follow the strategy based diagnostic steps for the reported symptoms. Record stored DTC's. Flash the new calibration from the table list following your Tech line terminal user instructions, and then clear all DTC's.
After flashing the new calibration, perform the EGR valve tests listed.
EGR Debris Purge and Function Check
1. Apply the parking brake. Place the transmission in Park, or Neutral for manual transmissions. Run the engine at 1500 RPM while using the Tech 1 in order to command the EGR valve to the 100% position, and then to the 0% position while monitoring the actual and commanded EGR position. This procedure is intended to clear foreign material from the EGR valve.
2. If the EGR valve's desired or commanded position agrees with the actual position, the EGR valve is functioning properly. If the EGR valve's desired or commanded position does not agree with the actual position, repeat step 1.
3. If the second attempt to clear foreign material from the EGR valve is not successful, follow the procedures in the Driveability and Emissions Service Manual for diagnosis of DTC P1406 and/or any other stored DTC's.
4. After the driveability concerns have been diagnosed and corrected, perform EGR Road Test.
EGR Road Test
1. During the EGR Road Test, with the new flash calibration installed, you may notice increased EGR activity during light throttle maneuvers. This is normal because the programmed EGR valve stroking is designed to clean the valve of debris. During this stroking, the actual EGR valve position will momentarily exceed the desired position.
2. Verify that the owner's reported driveability concerns are corrected and the EGR valve is functioning normally. If so, return the vehicle to the owner. If not, refer to the appropriate Service Manual Diagnosis Procedure.
Important:
The calibrations listed are FLASH Calibrations and not available from GMSPO. Service FLASH calibrations are available starting with the CD ROM listed. All engine calibrations listed are associated with Operating System (PH level software) P/N 16250645.
Warranty Information
For vehicles repaired under warranty, use:
Labor Operation Description Labor Time
J6355 Reprogramming Use Published Labor
Operation Time